The present invention provides a method for the production of a copolymer based on vinyl aromatic compounds and conjugated dienes with low cold flow, improved processability and good mechanical properties.
Methods for the production of rubber-elastic diene polymers, in which an improved cold flow is obtained by treating the diene polymers with specific sulfur halides, are known from DE-A 4,436,059 and DE-A 1,260,794. xe2x80x9cCold flowxe2x80x9d refers to the property by which polymers deform under the influence of small but constant forces generated by their own weight. This property is particularly troublesome in terms of the storage of elastomers and involves considerable technical complexity and financial outlay.
The object of the present invention is to reduce cold flow in vinyl aromatic diene copolymers and in doing so, to improve the processability and mechanical properties of the diene copolymers at the same time.
Therefore, the present invention provides a method for the production of copolymers based on vinyl aromatic compounds and conjugated dienes, characterized in that after polymerization the copolymers are treated with multi-functional sulfur halides in the presence of a catalyst at temperatures in the range from 20 to 130xc2x0 C.
Examples of vinyl aromatic compounds that can be used to formulate the copolymers include: styrene, p-methyl styrene, xcex1-methyl styrene, 3,5-dimethyl styrene, vinyl naphthalene, p-tert-butyl styrene, divinyl benzene and diphenyl ethylene.
1,3-butadiene, isoprene, 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene, 1-phenyl-1,3-butadiene, 1,3-hexadiene and 1,3-heptadiene can be used as conjugated dienes in the method according to the present invention.
Such vinyl aromatic diene copolymers are knownxe2x80x94as is their production by emulsion polymerization or anionic polymerizationxe2x80x94and are described, for example, in Ullmann""s Encyclopedia of Industrial Chemistry, 6th Edition, 1999 Electronic Release, 1999 Wiley-VCH, Weinheim, notably in the chapters xe2x80x9cRubber, 3. Synthetic, 2.1 Emulsion Styrene-Butadiene Rubber (E-SBR) and 3.1.1 Solution 1,3-Butadiene-Styrene Rubber (S-SBR) and Styrene-Isoprene-Butadiene Rubber (S-SIBR).
Emulsion polymerization is performed, as is known, by polymerization of the monomers thereby forming the copolymers in the presence of a radical initiator, e.g., a redox system, and a regulator, e.g., a dodecyl mercaptan, along with an emulsifier in an aqueous solvent and a short-stop reagent for short-stopping.
Accordingly, the anionic polymerization is performed in the presence of an alkali metal-based initiator, e.g., n-butyl lithium, in a hydrocarbon as solvent. Known randomizers, e.g., potassium alcoholates, and control agents for the polymer microstructure, e.g., ether or tert-amine, can additionally be used.
After polymerization of the vinyl aromatic diene copolymers, the copolymers are treated, according to the present invention, with multi-functional sulfur halides in the presence of a catalyst.
To this end, the sulfur halides can be mixed with the copolymers in a compounder or on a roll. A simpler method, however, particularly in the case of solution polymerization, is to meter the sulfur halides into the copolymer solution. This method can be performed especially, with ease and without the need for complex equipment, since such sulfur halides are soluble in the conventional polymerization solvents such as heptane, hexane, pentane, benzene, toluene and/or cyclohexane, and any sulfur halide that is not reacted, can be removed during the recovery process.
As mentioned above, the vinyl aromatic diene copolymers are treated with the multi-functional sulfur halides in the presence of a catalyst.
Suitable catalysts are Lewis acids, such as boron trifluoride, zinc dichloride, triethyl aluminum, aluminum trichloride, aluminum tribromide, tin tetrachloride and titanium tetrachloride, preferably aluminum trichloride, and tert-amines, such as, diisopropyl ethylamine, diazabicycloundecane and triethylamine.
Disulfur dichloride, sulfur dichloride, thionyl chloride, polysulfur dichloride, disulfur dibromide or polysulfur dibromide, preferably disulfur dichloride, can be used as multi-functional sulfur halides in the method according to the present invention.
The sulfur halides can be used either alone or combined as a mixture, as can the previously mentioned catalysts.
The multi-functional sulfur halides are generally used in the method according to the present invention in quantities of 0.005 to 5.0, preferably 0.01 to 0.5 wt. %, relative to the mass of the copolymer. The quantity of catalyst is generally from 0.05 to 5.0, preferably 0.1 to 2.0 wt. %, relative to the mass of the copolymer.
The method according to the present invention is preferably performed at temperatures in the range from 25 to 120xc2x0 C. On completion of the treatment of the vinyl aromatic diene copolymers with the multi-functional sulfur halides, the vinyl aromatic diene copolymers are recovered by mixing the solution with a stabilizer, such as, 2,6-di-tert-butyl-4-methylphenol and Araldite(copyright) DY 026SP from BASF AG, and isolating the vinyl aromatic diene copolymer by feeding the solution into alcohol, such as ethanol or isopropanol, or boiling water.
What is particularly surprising about the method according to the present invention is that not only the cold flow but also the processability of the diene copolymers and the mechanical properties of the vulcanizates manufactured therefrom were able to be improved. The copolymers according to the present invention can thus, be used in the manufacture of all types of moldings, especially tire components such as side walls and treads.
Indeed, DE-A 1,260,794, which is referred to above, teaches that in the production of rubber-elastic diene polymers, their cold flow can be improved by treatment with sulfur halides without impairing their processability and without influencing the mechanical properties of the vulcanizates (see column 1, lines 25-30). However, it cannot be construed from the cited prior art that by appropriate treatment of vinyl aromatic diene copolymers with sulfur halides in the presence of a catalyst, not only the cold flow but also their processability and the mechanical properties of the vulcanizates manufactured therefrom can be improved. As has been outlined above, this is to be judged as surprising on the basis of the known prior art.
The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.